WASHINGTON — In trying to decide whether the Environmental Protection Agency has the authority under two permitting programs to regulate greenhouse gas emissions from stationary sources such as power plants, the Supreme Court on Monday faced what Justice Elena Kagan called “the conundrum here.”

One part of the Clean Air Act, she said, seemed to require that such emissions be regulated. But another part set the emission thresholds so low that even schools and small businesses would be covered.

The agency’s solution was to raise those thresholds, and the resulting standards covered far fewer sources. That move was at the center of Monday’s arguments, and the justices seemed divided along ideological lines over whether it was a sensible accommodation or an impermissible exercise of executive authority.


Justice Anthony M. Kennedy, who may hold the decisive vote, made a point that did not bode well for the agency.

“I couldn’t find a single precedent that strongly supports your position,” he told the agency’s lawyer, Donald B. Verrilli Jr., the US solicitor general.

Verrilli said the solution to the conundrum was to allow the agency to exercise some discretion.

But Justice Samuel A. Alito Jr. suggested that the agency’s revision of numerical standards in a statute was without precedent in “the entire history of federal regulation.”

Even as the justices differed on the scope of the agency’s authority, though, they seemed to agree that the case before them was not particularly significant, for two reasons.

First, the narrow issue the Supreme Court agreed to address left in place the agency’s determinations that greenhouse gases present an urgent threat and that emissions from motor vehicles may be regulated.

Second, there seemed to be a consensus that the agency would retain other means to address emissions from stationary sources if the programs challenged in the case before the justices were struck down.


That last point made the case less important than it might have been, Justice Stephen G. Breyer told Peter Keisler, a lawyer representing groups challenging the regulations. Keisler said the two approaches were not identical, as the challenged one relied on state and local authorities while the other would set a national standard.

The immediate question in the case, Utility Air Regulatory Group v. EPA, centers on what to do about parts of the Clean Air Act that say two permitting programs cover all sources that can annually emit 100 or 250 tons of the relevant pollutant, a threshold that works for conventional air pollutants such as lead and carbon monoxide. But applying those thresholds to greenhouse gases, which are emitted in far greater amounts, would require the regulation of millions of sources of pollution.